In the development of the craniofacial complex cells acquire differentiated states and transmit those states faithfully to daughter cells. This proposal seeks to explain in molecular terms the stability of differentiation in replicating cells. Two questions are posed--(A) How does chromatin replicate and (B) Can keratinocytes in culture undergo pedifferentiation. (A) Chromatin replication: We seek to learn if there is a co-ordinated assembly of newly synthesized and pre-existing chromosomal proteins that might explain how the DNA-protein complexes in parental chromatin are faithfully reproduced in daughter chromatin. The experiments utilize chromatin isolation and fractionation techniques developed by the principal investigator. Four areas related to chromatin replication are explored: 1) the structure of nascent chromatin; 2) chromatin made in the absence of protein synthesis; 3) "repaired" chromatin (i.e., following damage to the DNA by known mutagens and carcinogens); and 4) chromatin synthesized when DNA is crosslinked to chromosomal proteins. (B) Keratinocyte redifferentiation. Single keratinocytes from oral mucosa have been cultured in vitro by the principal investigator and observed to form colonies of cells that resemble non-keratinizing stratified squamous epithelium. The stability of their differentiation is probed by culturing these keratinocytes with fibroblasts from keratinizing and non-keratinizing tissue and with agents known to block differentiation, i.e., bromodeoxyuridine. In addition, keratinocytes from oral dysplastic lesion will be cultured to learn if an altered state of differentiation is expressed in culture. The information derived from this study will help explain how permanent phenotypic changes could arise by epigenetic mechanisms and why dysmorphogenic syndromes and neoplasia might be the pathological counterpart of normal differentiation.